Bore holes, such as oil well bore holes, are usually drilled by the rotary method in which a drilling fluid is circulated in the bore hole during the drilling operation. The presence of a drilling fluid is required to act as coolant for the drill bit and drill stem, to act as carrier for the cuttings and to provide hydraulic pressure to prevent blowout. Subterranean formations through which the bore hole is being drilled often presents conditions which cause whole drilling fluid (commonly known as "mud") to be lost. Such conditions are generally classified as either of a macroporosity type, e.g. loosely consolidated sandy formations, or of a macroscopic fracture type, e.g. crevices directed away from the bore hole. The loss of whole mud via either of these two conditions is known as lost circulation.
Loss of whole mud is an extremely undesirable phenomenon as it can (1) lead to poor circulation and, therefore, less efficient removal of cuttings, (2) require additional cost in rig time, manpower and material to replenish the lost mud and restore circulation and, in extreme cases, (3) lead to insufficient downhole hydrostatic pressure and to a blowout.
Materials used to stop lost circulation are known as lost circulation controllers or additives. Current lost circulation controllers are generally divided into three different categories: fibers, flakes and granules. Each category is believed useful with respect to a specific type of formation. Materials which are in the form of fibers or flakes are presently used to seal macroporous formations. Examples of fibrous material commonly used includes cedar fibers and bagasse. Examples of flake material commonly used includes paper, mica and cottonseed hulls. Granular materials, such as ground walnut or pecan shells, are commonly used to plug fractures. Therefore, circulation control using presently known materials requires knowledge of the nature of the subterranean formation and/or the use of various types or combinations of material until mud circulation is substantially restored.
Tests have been devised to determine the usefulness of a particular material as a control additive with respect to each of the two types of lost circulation causing formations. The "slot test" is a standard test used to determine control properties with respect to fractures. This test observes the ability of a mud which is under pressure (simulating bore hole pressure) containing the control candidate to plug a slit of predetermined dimension. The "shot bed test" observes the ability of a mud to plug macroporous formations by having mud containing the control candidate to attempt to pass through a column of shot of standard size.
U.S. Pat. Nos. 2,943,679 and 2,943,680 describe the use of hard portions of certain nut shells which have been ground to a granular form as being useful as a lost circulation controller with respect to fractures. U.S. Pat. No. 3,629,102 discloses that certain specific mixtures of both fibrous and granular materials can be used as a lost circulation controller for both fracture and macroporosity loss.
It is highly desired to have a single material which can be used as universal lost circulation controller for both fracture and macroporosity loss. Such a material would alleviate the present requirement for detailed knowledge of the subterranean formation, stocking of different types of material for proper application and for application of different types of material, singly or in combination, to try to control fluid loss by trial and error.